Iron oxide based catalysts are widely used in the dehydrogenation of hydrocarbons. Iron oxide materials from which these catalyst may be prepared exist naturally as several minerals. These minerals include red, yellow, brown, and black iron oxide materials. For example, red iron oxide minerals are usually hematite (.alpha.-Fe.sub.2 O.sub.3), yellow iron oxide can be lepidocrocite (.gamma.-FeOOH or Fe.sub.2 O.sub.3.nH.sub.2 O) or goethite (.alpha.-FeOOH or Fe.sub.2 O.sub.3.nH.sub.2 O), brown iron oxide is maghemite (.gamma.-Fe.sub.2 O.sub.3), and black iron oxide is magnetite (Fe.sub.3 O.sub.4).
Synthetic hematite, goethite, lepidocrocite, maghemite, and magnetite are among the most important iron oxides for use in industrial applications. Synthetic hematite produced by calcination of synthetic goethite is most widely used to catalyze the conversion of ethylbenzene to styrene because these materials often have the highest purity (&gt;98% Fe.sub.2 O.sub.3).
U.S. Pat. Nos. 4,052,338; 4,098,723; 4,143,083; 4,144,197; and 4,152,300 all propose dehydrogenation catalysts comprising small amounts of oxidic compounds and rare earths added to iron-potassium oxide base catalysts. In each case, these components were blended, pelletized, and dried. The pellets were then calcined. Selectivity was consistent at approximately 92 mole % (for styrene) among these compositions at a 70% molar conversion of ethylbenzene to products. U.S. Pat. No. 5,023,225 proposed a catalyst with improved stability. In this patent, prior to catalyst formation, a yellow iron oxide was heated in the presence of a small amount of a chromium compound until the yellow iron oxide was converted to red iron oxide.
It has now been found that iron oxide based catalysts with enhanced catalytic properties can be prepared by predoping iron oxide compounds under certain conditions. The catalysts produced from such predoped iron oxides display particularly noteworthy selectivity improvements over iron oxide based catalysts which have not been so predoped.